Silicone rubber reinforced with reinforced precipitated silica

ABSTRACT

Elastomeric compositions contain crosslinked poly(diorganosiloxane) and reinforced precipitated silica having, on a coating-free and impregnant-free basis, a surface area of from about 220 to about 340 square meters per gram, a pore diameter at the maximum of the volume pore size distribution function of from about 9 to about 20 nanometers, and a total intruded volume of from about 2.6 to about 4.4 cubic centimeters per gram.

This is a divisional of application Ser. No. 07/541,679, filed Jun. 21,1990, now U.S. Pat. No. 5,094,826.

Many different precipitated silicas are known and have been used in awide variety of applications. Precipitated silicas are most commonlyproduced by precipitation from an aqueous solution of sodium silicateusing a suitable acid such as sulfuric acid, hydrochloric acid, and/orcarbon dioxide. Processes for producing precipitated silicas aredescribed in detail in U.S. Pat. Nos. 2,657,149; 2,940,830; and4,681,750, the entire disclosures of which are incorporated herein byreference, including especially the processes for making precipitatedsilicas and the properties of the products.

Although both are silicas, it is important to distinguish precipitatedsilica from silica gel inasmuch as these different materials havedifferent properties. Reference in this regard is made to R. K. Iler,The Chemistry of Silica, John Wiley & Sons, New York (1979), Library ofCongress Catalog No. QD 181.S6144. Note especially pages 15-29, 172-176,218-233, 364-365, 462-465, 554-564, and 578-579, the entire disclosuresof which are incorporated herein by reference. Silica gel is usuallyproduced commercially at low pH by acidifying an aqueous solution of asoluble metal silicate, customarily sodium silicate, with acid. The acidemployed is generally a strong mineral acid such as sulfuric acid orhydrochloric acid although carbon dioxide is sometimes used. Inasmuch asthere is essentially no difference in density between the gel phase andthe surrounding liquid phase while the viscosity is low, the gel phasedoes not settle out, that is to say, it does not precipitate. Silicagel, then, may be described as a non-precipitated, coherent, rigid,three-dimensional network of contiguous particles of colloidal amorphoussilica. The state of subdivision ranges from large, solid masses tosubmicroscopic particles, and the degree of hydration from almostanhydrous silica to soft gelatinous masses containing on the order of100 parts of water per part of silica by weight, although the highlyhydrated forms are only rarely used.

Precipitated silica is usually produced commercially by combining anaqueous solution of a soluble metal silicate, ordinarily alkali metalsilicate such as sodium silicate, and an acid so that colloidalparticles will grow in weakly alkaline solution and be coagulated by thealkali metal ions of the resulting soluble alkali metal salt. Variousacids may be used, including the mineral acids and/or carbon dioxide. Inthe absence of a coagulant, silica is not precipitated from solution atany pH. The coagulant used to effect precipitation may be the solublealkali metal salt produced during formation of the colloidal silicaparticles, it may be added electrolyte such as a soluble inorganic ororganic salt, or it may be a combination of both.

Precipitated silica, then, may be described as precipitated aggregatesof ultimate particles of colloidal amorphous silica that have not at anypoint existed as macroscopic gel during the preparation. The sizes ofthe aggregates and the degree of hydration may vary widely.

Precipitated silica powders differ from silica gels that have beenpulverized in ordinarily having a more open structure, that is, a higherspecific pore volume. However, the specific surface area of precipitatedsilica as measured by the Brunauer, Emmett, Teller (BET) method usingnitrogen as the adsorbate, is often lower than that of silica gel.

Variations in the parameters and/or conditions during production resultin variations in the types of precipitated silicas produced. Althoughthey are all broadly precipitated silicas, the types of precipitatedsilicas often differ significantly in physical properties and sometimesin chemical properties. These differences in properties are importantand often result in one type being especially useful for a particularpurpose but of marginal utility for another purpose, whereas anothertype is quite useful for that other purpose but only marginally usefulfor the first purpose.

Reinforcement of precipitated silica, that is, the deposition of silicaon aggregates of previously precipitated silica, is itself known. It hasnow been found, however, that by controlling the conditions of silicaprecipitation and multiple reinforcement steps, new silicas may beproduced having properties that make them especially useful forclarifying beer and for reinforcing silicone rubbers. They also may beused for many of the purposes for which other types of precipitatedsilicas have been used. For example they may be used as reinforcingfillers for styrene-butadiene rubber and other organic rubbers. They maybe used as fillers and extenders in toothpaste; as carriers forvitamins; as paper extenders and brighteners; and in a multitude ofother uses.

Although it is not desired to be bound by any theory, it is believedthat as precipitated silica is dried, the material shrinks;consequently, pore diameters are reduced, surface area is reduced, andthe void volume is reduced. It is further believed that by sufficientlyreinforcing the silica prior to drying, a more open structure isobtained after drying. Irrespective of theory, the reinforcedprecipitated silica of the present invention has, on balance, largerpore diameters and a larger total intruded volume for the surface areaobtained than is the case for previous precipitated silicas, whether ornot reinforced.

Accordingly, one embodiment of the invention is a process for producingreinforced amorphous precipitated silica having, on a coating-free andimpregnant-free basis, a surface area of from about 220 to about 340square meters per gram, a pore diameter at the maximum of the volumepore size distribution function of from about 9 to about 20 nanometers,and a total intruded volume of from about 2.6 to about 4.4 cubiccentimeters per gram, the process comprising: (a) establishing aninitial aqueous alkali metal silicate solution containing from about 0.5to about 4 weight percent SiO₂ and having an SiO₂ :M₂ O molar ratio offrom about 1.6 to about 3.9; (b) over a period of at least about 20minutes and with agitation, adding acid to the initial aqueous alkalimetal silicate solution at a temperature below about 50° C. toneutralize at least about 60 percent of the M₂ O present in the initialaqueous alkali metal solution and thereby to form a first reactionmixture; (c) over a period of from about 115 to about 240 minutes, withagitation, and at a temperature of from about 80° C. to about 95° C.,substantially simultaneously adding to the first reaction mixture: (1)additive aqueous alkali metal silicate solution, and (2) acid, therebyto form a second reaction mixture wherein the amount of the additiveaqueous alkali metal silicate solution added is such that the amount ofSiO₂ added is from about 0.5 to about 2 times the amount of SiO₂ presentin the initial aqueous alkali metal silicate solution established instep (a) and wherein the amount of the acid added is such that at leastabout 60 percent of the M₂ O contained in the additive aqueous alkalimetal silicate solution added during the simultaneous addition isneutralized; (d) adding acid to the second reaction mixture withagitation at a temperature of from about 80° C. to about 95° C. to forma third reaction mixture having a pH below 7; (e) aging the thirdreaction mixture with agitation at a pH below 7 and at a temperature offrom about 80° C. to about 95° C. for a period of from about 1 to about120 minutes; (f) with agitation and at a temperature of from about 80°C. to about 95° C., adding to the aged third reaction mixture additiveaqueous alkali metal silicate solution to form a fourth reaction mixturehaving a pH of from about 7.5 to about 9; (g) forming a fifth reactionmixture by adding to the fourth reaction mixture with agitation and at atemperature of from about 80° C. to about 95° C., a further quantity ofadditive aqueous alkali metal silicate solution and adding acid asnecessary to maintain the pH at from about 7.5 to about 9 during theaddition of the further quantity of the additive aqueous alkali metalsilicate solution, wherein: (1) the amount of the additive aqueousalkali metal silicate solution added in steps (f) and (g) is such thatthe amount of SiO₂ added in steps (f) and (g) is from about 0.05 toabout 0.75 times the amount of SiO₂ present in the third reactionmixture, and (2) the additive aqueous alkali metal silicate solution isadded in steps (f) and (g) over a collective period of at least about 40minutes; (h) aging the fifth reaction mixture with agitation at atemperature of from about 80° C. to about 95° C. for a period of fromabout 5 to about 60 minutes; (i) adding acid to the aged fifth reactionmixture with agitation at a temperature of from about 80° C. to about95° C. to form a sixth reaction mixture having a pH below 7; (j) agingthe sixth reaction mixture with agitation at a pH below 7 and at atemperature of from about 80° C. to about 95° C. for a period of atleast about 1 minute; (k) separating reinforced precipitated silica frommost of the liquid of the aged sixth reaction mixture; (1) washing theseparated reinforced precipitated silica with water; and (m) drying thewashed reinforced precipitated silica, wherein: (n) the alkali metalsilicate is lithium silicate, sodium silicate, potassium silicate, or amixture thereof; and (o) M is lithium, sodium, potassium, or a mixturethereof.

Optionally, prior to step (c) the first reaction mixture is aged withagitation at a temperature of from about 30° C. to about 95° C. for aperiod of from about 5 to about 180 minutes.

The composition of the initial aqueous alkali metal silicate solutionestablished in step (a) may vary widely. Generally the initial aqueousalkali metal silicate solution comprises from about 0.5 to about 4weight percent SiO₂. In many cases the initial aqueous alkali metalsilicate solution comprises from about 1 to about 3 weight percent SiO₂.From about 1.5 to about 2.5 weight percent SiO₂ is preferred. Usuallythe initial aqueous alkali metal silicate solution has an SiO₂ :M₂ Omolar ratio of from about 1.6 to about 3.9. Often the SiO₂ :M₂ O molarratio is from about 2.5 to about 3.6. Preferably the SiO₂ :M₂ O molarratio is from about 2.9 to about 3.6. Often the SiO₂ :M₂ O molar ratiois from about 3.2 to about 3.3.

The composition of the additive aqueous alkali metal silicate solutionmay also vary widely. Usually the additive aqueous alkali metal silicatesolution comprises from about 2 to about 30 percent by weight SiO₂.Often the additive aqueous alkali metal silicate solution comprises fromabout 10 to about 15 percent by weight SiO₂. From about 12 to about 13weight percent SiO₂ is preferred. Frequently the additive aqueous alkalimetal silicate solution has an SiO₂ :M₂ O molar ratio of from about 1.6to about 3.9. In many cases the SiO₂ :M₂ O molar ratio is from about 2.5to about 3.6. Preferably the SiO₂ :M₂ O molar ratio is from about 2.9 toabout 3.6. Often the SiO₂ :M₂ O molar ratio is from about 3.2 to about3.3. Additive aqueous alkali metal silicate solution having the samecomposition may be used throughout the various silicate additions, oradditive aqueous alkali metal silicate solutions having differingcompositions may be used in different silicate addition steps.

The acid used in the process may also vary widely. In general, the acidadded in steps (b), (c), and (g) should be strong enough to neutralizealkali metal silicate and cause precipitation of silica. The acid addedin steps (d) and (i) should be strong enough to reduce the pH to desiredvalues within the specified ranges. The acid used in the various acidaddition steps may be the same or different, but preferably it is thesame. A weak acid such as carbonic acid produced by the introduction ofcarbon dioxide to the reaction mixture may be used for precipitation ofsilica, but a stronger acid must be used in steps (d) and (i) when it isdesired to reduce the pH to values below 7. It is preferred to usestrong acid throughout the process. Examples of the strong acids includesulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, andacetic acid. The strong mineral acids such as sulfuric acid,hydrochloric acid, nitric acid, and phosphoric acid are preferred;sulfuric acid is especially preferred.

The acid addition of step (b) is made over a period of at least about 20minutes. Frequently the acid addition of step (b) is made over a periodof from about 20 to about 60 minutes. From about 26 to about 32 minutesis preferred.

The temperature of the reaction mixture during the acid addition of step(b) is below about 50° C. From about 30° C. to about 40° C. ispreferred.

At least about 60 percent of the M₂ O present in the initial aqueousalkali metal silicate solution is neutralized during the acid additionof step (b). As much as 100 percent of the M₂ O may be neutralized ifdesired. Preferably from about 75 to about 85 percent of the M₂ O isneutralized.

The additions made in step (c) are made over a period of from about 115to about 240 minutes. Preferably the additions are made over a period offrom about 115 to about 125 minutes.

The temperature of the reaction mixture during the additions of step (c)is from about 80° C. to about 95° C. From about 90° C. to about 95° C.is preferred.

In step (c), the amount of additive aqueous alkali metal silicate addedis such that the amount of SiO₂ added is from about 0.5 to about 2 timesthe amount of SiO₂ present in the initial aqueous alkali metal silicatesolution established in step (a). From about 0.9 to about 1.1 times theSiO₂ present in the initial aqueous alkali metal silicate solution ispreferred.

The amount of acid added in step (c) is such that at least about 60percent of the M₂ O contained in the additive aqueous alkali metalsilicate solution added in step (c) is neutralized. As much as 100percent of such M₂ O may be neutralized if desired. Preferably fromabout 75 to about 85 percent of the M₂ O is neutralized.

The temperature of the reaction mixture during the acid addition of step(d) is from about 80° C. to about 95° C. From about 90° C. to about 95°C. is preferred.

In step (d), the acid is added such that the pH of the third reactionmixture is below 7. Often the pH is from about 2.5 to below 7. A pH offrom about a to about 5 is preferred.

Similarly, the third reaction mixture is aged in step (e) at a pH below7. Often the pH is from about 2.5 to below 7. A pH of from about 4 toabout 5 is preferred.

The temperature of the third reaction mixture during the aging of step(e) is from about 80° C. to about 95° C. From about 90° C. to about 95°C. is preferred.

The aging in step (e) is for a period of from about 1 to about 120minutes. In many cases the third reaction mixture is aged for a periodof from about 15 to about 120 minutes. A period of from about 15 toabout 30 minutes is preferred.

The temperature of the reaction mixture during the addition of additiveaqueous alkali metal silicate solution in step (f) is from about 80° C.to about 95° C. From about 90° C. to about 95° C. is preferred.

The pH of the fourth reaction mixture formed in step (f) is from about7.5 to about 9. A pH of from about 8 to about 9 is preferred.

Acid is added in step (g) as necessary to maintain the pH of thereaction mixture at from about 7.5 to about 9 during the addition of thefurther quantity of additive aqueous alkali metal silicate solution. ApH of from about 8 to about 9 is preferred.

The amount of additive aqueous alkali metal silicate solution added insteps (f) and (g) is such that the amount of SiO₂ added in steps (f) and(g) is from about 0.05 to about 0.75 times the amount of SiO₂ present inthe third reaction mixture. Preferably the amount of additive aqueousalkali metal silicate solution added in steps (f) and (g) is such thatthe amount of SiO₂ added in steps (f) and (g) is from about 0.25 toabout 0.45 times the amount of SiO₂ present in the third reactionmixture.

The additive alkali metal silicate solution is added in steps (f) and(g) over a collective period of at least about 40 minutes. A collectiveperiod of from about 40 to about 240 minutes is often employed. Acollective period of from about 70 to about 100 minutes is preferred.

The temperature of the fifth reaction mixture during the aging of step(h) is from about 80° C. to about 95° C. From about 90° C. to about 95°C. is preferred.

In step (h), the fifth reaction mixture is aged for a period of fromabout 5 to about 60 minutes. A period of from about 30 to about 60minutes is preferred.

The temperature of the reaction mixture during the acid addition of step(i) is from about 80° C. to about 95° C. From about 90° C. to about 95°C. is preferred.

In step (i), the acid is added such that the pH of the sixth reactionmixture is below 7. Often the pH is from about 2.5 to below 7. A pH offrom about 4 to about 5 is preferred.

The sixth reaction mixture is aged in step (j) at a pH below 7. In manycases the pH is from about 2.5 to below 7. A pH of from about 4 to about5 is preferred.

The temperature of the sixth reaction mixture during the aging of step(j) is from about 80° C. to about 95° C. From about 90° C. to about 95°C. is preferred.

In step (j), the sixth reaction mixture is aged for a period of at leastabout 1 minute. Often the aging period is at least about 30 minutes. Anaging period of at least about 50 minutes is preferred.

The separation of step (k) may be accomplished by one or more techniquesfor separating solids from liquid such as, for example, filtration,centrifugation, decantation, and the like.

The washing of step (1) may be accomplished by any of the proceduresknown to the art for washing solids. Examples of such procedures includepassing water through a filter cake, and reslurring the reinforcedprecipitated silica in water followed by separating the solids from theliquid. One washing cycle or a succession of washing cycles may beemployed as desired. The primary purpose of washing is to remove saltformed by the various neutralizations to desirably low levels. Usuallythe reinforced precipitated silica is washed until the concentration ofsalt in the dried reinforced precipitated silica is less than or equalto about 2 percent by weight. Preferably the reinforced precipitatedsilica is washed until the concentration of salt is less than or equalto about 0.2 percent by weight.

The drying of step (m) may also be accomplished by one or more knowntechniques. For example, the reinforced precipitated silica may be driedin an air oven or in a vacuum oven. Preferably the reinforcedprecipitated silica is dispersed in water and spray dried in a column ofhot air. The temperature at which drying is accomplished is notcritical, but the usual practice is to employ temperatures of at least70° C. Generally the drying temperature is less than about 700° C. Inmost cases drying is continued until the reinforced precipitated silicahas the characteristics of a powder. Ordinarily the dried reinforcedprecipitated silica is not absolutely anhydrous but contains bound water(from about 2 to about 5 weight percent) and adsorbed water (from about1 to about 7 weight percent) in varying amounts, the latter dependingpartly upon the prevailing relative humidity. Adsorbed water is thatwater which is removed from the silica by heating at 105° C. for 24hours at atmospheric pressure in a laboratory oven. Bound water is thatwater which is removed by additionally heating the silica at calcinationtemperatures, for example, from about 1000° C. to about 1200° C.

Another optional step which may be employed is size reduction. Sizereduction techniques are themselves well known and may be exemplified bygrinding and pulverising. Particularly preferred is fluid energy millingusing air or superheated steam as the working fluid. Fluid energy millsare themselves well known. See, for example, Perry's Chemical Engineers'Handbook, 4th Edition, McGraw-Hill Book Company, New York, (1963),Library of Congress Catalog Card Number 6113168, pages 8-42 and 8-43;McCabe and Smith, Unit Operations of Chemical Engineering, 3rd Edition,McGraw-Hill Book Company, New York (1976), ISBN 0-07-044825-6, pages 844and 845; F. E. Albus, "The Modern Fluid Energy Mill", ChemicalEngineering Progress, Volume 60, No. 6 (June 1964), pages 102-106, theentire disclosures of which are incorporated herein by reference. Influid energy mills the solid particles are suspended in a gas stream andconveyed at high velocity in a circular or elliptical path. Somereduction occurs when the particles strike or rub against the walls ofthe confining chamber, but most of the reduction is believed to becaused by interparticle attrition.

The degrees of agitation used in the various steps of the invention mayvary considerably. The agitation employed during the addition of one ormore reactants should be at least sufficient to provide a thoroughdispersion of the reactants and reaction mixture so as to avoid morethan trivial locally high concentrations of reactants and to ensure thatsilica deposition occurs substantially uniformly thereby avoidinggellation on the macro scale. The agitation employed during aging shouldbe at least sufficient to avoid settling of solids to ensure that silicadeposition occurs substantially uniformly throughout the mass of silicaparticles rather than preferentially on those particles at or near thetop of a settled layer of particles. The degrees of agitation may, andpreferably are, greater than these minimums. In general, vigorousagitation is preferred.

Yet another optional step which may be employed is treating thereinforced precipitated silica with one or more materials which coat,partially coat, impregnate, and/or partially impregnate the silica. Manymaterials may be used for this purpose. In general, the type of materialused depends upon the effect desired. Most often the materials areorganic polymers. Examples of suitable materials include hydrocarbonoils, polyesters, polyamides, phenolic resins, aminoplast resins,polysiloxanes, polysilanes, and the like. The treatment step may beaccomplished at any convenient time during or after formation of thereinforced precipitated silica.

A preferred embodiment within the first embodiment of the invention is aprocess for producing reinforced amorphous precipitated silica having,on a coating-free and impregnant-free basis, a surface area of fromabout 220 to about 340 square meters per gram, a pore diameter at themaximum of the volume pore size distribution function of from about 13to about 18 nanometers, and a total intruded volume of from about 3 toabout 4.4 cubic centimeters per gram, the process comprising: (a)establishing an initial aqueous alkali metal silicate solutioncontaining from about 0.5 to about 4 weight percent SiO₂ and having anSiO₂ :M₂ O molar ratio of from about 1.6 to about 3.9; (b) over a periodof at least about 20 minutes and with agitation, adding acid to theinitial aqueous alkali metal silicate solution at a temperature of fromabout 30° C. to about 40° C. to neutralize from about 75 to about 85percent of the M₂ O present in the initial aqueous alkali metal solutionand to form a first reaction mixture; (c) over a period of from about115 to about 125 minutes, with agitation, and at a temperature of fromabout 90° C. to about 95° C., substantially simultaneously adding to thefirst reaction mixture: (1) additive aqueous alkali metal silicatesolution, and (2) acid, to form a second reaction mixture wherein theamount of the additive aqueous alkali metal silicate solution added issuch that the amount of SiO₂ added is from about 0.9 to about 1.1 timesthe amount of SiO₂ present in the initial aqueous alkali metal silicatesolution established in step (a) and wherein the amount of the acidadded is such that from about 75 to about 85 percent of the M₂ Ocontained in the additive aqueous alkali metal silicate solution addedduring the simultaneous addition is neutralized; (d) adding acid to thesecond reaction mixture with agitation at a temperature of from about90° C. to about 95° C. to form a third reaction mixture having a pH offrom about 4 to about 5; (e) aging the third reaction mixture withagitation at a temperature of from about 90° C. to about 95° C. for aperiod of from about 15 to about 30 minutes; (f) with agitation and at atemperature of from about 90° C. to about 95° C., adding to the agedthird reaction mixture additive aqueous alkali metal silicate solutionto form a fourth reaction mixture having a pH of from about 8 to about9; (g) forming a fifth reaction mixture by adding to the fourth reactionmixture with agitation and at a temperature of from about 90° C. toabout 95° C., a further quantity of additive aqueous alkali metalsilicate solution and adding acid as necessary to maintain the pH atfrom about 8 to about 9 during the addition of the further quantity ofthe additive aqueous alkali metal silicate solution, wherein: (1) theamount of the additive aqueous alkali metal silicate solution added insteps (f) and (g) is such that the amount of SiO₂ added in steps (f) and(g) is from about 0.25 to about 0.45 times the amount of SiO₂ present inthe third reaction mixture, and (2) the additive aqueous alkali metalsilicate solution is added in steps (f) and (g) over a collective periodof from about 70 to about 100 minutes; (h) aging the fifth reactionmixture with agitation at a temperature of from about 90° C. to about95° C. for a period of from about 30 to about 60 minutes; (i) addingacid to the aged fifth reaction mixture with agitation at a temperatureof from about 90° C. to about 95° C. to form a sixth reaction mixturehaving a pH of from about 4 to about 5; (j) aging the sixth reactionmixture with agitation at a temperature of from about 90° C. to about95° C. for a period of at least about 50 minutes; (k) separatingreinforced precipitated silica from most of the liquid of the aged sixthreaction mixture; (1) washing the separated reinforced precipitatedsilica with water; and (m) drying the washed reinforced precipitatedsilica, wherein: (n) the alkali metal silicate is lithium silicate,sodium silicate, potassium silicate, or a mixture thereof; and (o) M islithium, sodium, potassium, or a mixture thereof.

It is understood that one or more ranges in the preferred embodiment maybe used in lieu of the corresponding broader range or ranges in thebroader first embodiment of the invention.

A further embodiment of the invention is reinforced amorphousprecipitated silica having, on a coating-free and impregnant-free basis,a surface area of from about 220 to about 340 square meters per gram, apore diameter at the maximum of the volume pore size distributionfunction of from about 9 to about 20 nanometers, and a total intrudedvolume of from about 2.6 to about 4.4 cubic centimeters per gram. Theconcurrence of all three of these properties is essential to thereinforced precipitated silica of the present invention.

As used in the present specification and claims, the surface area of thereinforced amorphous precipitated silica is the surface area determinedby the Brunauer, Emmett, Teller (BET) method according to ASTM C 819-77using nitrogen as the adsorbate but modified by outgassing the systemand the sample for one hour at 180° C. The surface area is from about220 to about 340 square meters per gram. In many cases the surface areais from about 220 to about 300 square meters per gram. From about 220 toabout 270 square meters per gram is preferred. ASTM C 819-77 is, in itsentirety, incorporated herein by reference.

The volume average pore size distribution function of the reinforcedamorphous precipitated silica is determined by mercury porosimetry usingan Autoscan mercury porosimeter (Quantachrome Corp.) in accordance withthe accompanying operating manual. In operating the porosimeter, a scanis made in the high pressure range (from about 103 kilopascals absoluteto about 227 megapascals absolute). The volume pore size distributionfunction is given by the following equation: ##EQU1## where: D_(v) (d)is the volume pore size distribution function, usually expressed in cm³/(μm·g);

d is the pore diameter, usually expressed in μm;

P is the pressure, usually expressed in pounds per square inch,absolute; and

V is the pore volume per unit mass, usually expressed in cm³ /g.

Dv(d) is determined by taking ΔV/ΔP for small values of ΔP from either aplot of V versus P or preferably from the raw data. Each value of ΔV/ΔPis multiplied by the pressure at the upper end of the interval anddivided by the corresponding pore diameter. The resulting value isplotted versus the pore diameter. The value of the pore diameter at themaximum of the volume pore size distribution function is then taken fromthe plotted graph. Numerical procedures may be used rather thangraphical when desired. For the reinforced amorphous precipitated silicaof the present invention the pore diameter at the maximum of the volumepore size distribution function is from about 9 to about 20 nanometers.Preferably the pore diameter at the maximum of the function is fromabout 13 to about 18 nanometers.

In the course of determining the volume average pore diameter by theabove procedure, the maximum pore radius detected is sometimes noted.The maximum pore diameter is twice the maximum pore radius.

The total intruded volume is the total volume of mercury which isintruded into the reinforced amorphous precipitated silica during thehigh pressure scan described above divided by the mass of the reinforcedamorphous precipitated silica constituting the sample under test. Thetotal intruded volume of the reinforced amorphous precipitated silica isfrom about 2.6 to about 4.4 cubic centimeters per gram. Preferably thetotal intruded volume is from about 3 to about 4.4 cubic centimeters pergram.

The reinforced amorphous precipitated silica may be in the form ofaggregates of ultimate particles, agglomerates of aggregates, or acombination of both. Ordinarily, less than about 10 percent by weight ofthe reinforced amorphous precipitated silica has gross particle sizesgreater than about 80 micrometers as determined by use of a Model TAIICoulter counter (Coulter Electronics, Inc.) according to ASTM C 690-80but modified by stirring the precipitated silica for 10 minutes inIsoton II electrolyte (Curtin Matheson Scientific, Inc.) using afour-blade, 4.5 centimeter diameter propeller stirrer. In many casesless than about 10 percent by weight of the reinforced amorphousprecipitated silica has gross particle sizes greater than about 40micrometers. When size reduction is employed usually less than about 10percent by weight of the reinforced amorphous precipitated silica hasgross particle sizes greater than about 20 micrometers. Preferably lessthan about 10 percent by weight of the reinforced amorphous precipitatedsilica has gross particle sizes greater than about 10 micrometers. Whenless than about 10 percent by weight of the reinforced amorphousprecipitated silica has gross particle sizes greater than about 20micrometers, it is preferred that the median gross particle size be lessthan about 5 micrometers. When less than about 10 percent by weight ofthe reinforced amorphous precipitated silica has gross particle sizesgreater than about 10 micrometers, it is preferred that the median grossparticle size be less than about 2 micrometers. It is expected that insome usages such as fillers for battery separators, microporousmaterials, and rubbers, the sizes of reinforced amorphous precipitatedsilica aggregates will be reduced during processing of the ingredientsto prepare the final articles. Accordingly, the distribution of grossparticle sizes in such articles may be smaller than in the rawreinforced amorphous precipitated silica itself. ASTM C 690-80 is, inits entirety, incorporated herein by reference.

The average ultimate particle size of the reinforced amorphousprecipitated silica (irrespective of whether or not the ultimateparticles are aggregated and/or agglomerated) is usually less than about0.1 micrometer as determined by transmission electron microscopy. Oftenthe average ultimate particle size is less than about 0.05 micrometer.Preferably the average ultimate particle size is less than about 0.03micrometer.

The neutralization of alkali metal silicate with acid to produce thereinforced amorphous precipitated silica of the invention also producesalkali metal salt of the acid(s) used for neutralization as by-product.It is preferred that the amount of such salt associated with thereinforced amorphous precipitated silica product be low. When thereinforced amorphous precipitated silica is separated from the liquid ofthe aged sixth reaction mixture, most of the salt is removed with theliquid. Further amounts of salt may conveniently be removed by washingthe separated reinforced precipitated silica with water. In general, thegreater the amount of water used for washing, the lower will be the saltcontent of the final dried product. It is usually preferred thatreinforced amorphous precipitated silica contain less than about 1percent by weight alkali metal salt. Frequently the reinforced amorphousprecipitated silica contains less than about 0.5 percent by weightalkali metal salt. It is often particularly preferred that thereinforced amorphous precipitated silica contain less than about 0.2percent by weight alkali metal salt.

The reinforced amorphous precipitated silica of the present inventionmay be hydrophilic or it may be hydrophobic. Hydrophobic precipitatedsilica may be produced either by treating the hydrophilic precipitatedsilica with a hydrophobic coating or impregnating composition, or byadding hydrophobes to the precipitated but undried silica. Examples ofmethods for treating hydrophilic precipitated silica to render ithydrophobic include: (1) treatment after the manner of Iler, U.S. Pat.No. 2,657,149; (2) treatment with silane containing hydrophobic group(s)and hydrolyzable group(s); (3) treatment with hexamethyldisilazane; and(4) treatment with silicone oil such as for instance, trimethylend-blocked poly(dimethylsiloxane) or silanol terminatedpoly(dimethylsiloxane). In general, hydrophobic precipitated silica ismore compatible with silicone rubber than hydrophilic precipitatedsilica. Treatment of hydrophilic precipitated silica with silicone oilis the preferred method for producing hydrophobic precipitated silicabecause it has been found that the silicone oil not only improvescompatibility of the precipitated silica with silicone rubber, but italso often results in silicone rubbers having lower hardness. Thesilicone oils are themselves well known and are poly(organosiloxanes)which are flowable. The molecular weights of the silicone oils may varywidely, but usually they are in the range of from about 150 to about450,000. Often the molecular weight is in the range of from about 200 toabout 100,000. From about 400 to about 4500 is preferred. Although it isnot desired to be bound by any theory, it is believed that the surfacesof the precipitated silica particles have hydroxyl groups attached tosilicon atoms. When the silicone oil is applied to the particles andheated, it is further believed that at least some of the existingterminal groups and/or some of the terminal groups created by chainscission of the silicone oil condense with at least some of thehydroxyls of the particle surface to form siloxane bonds and evolvewater, alcohol, or some other compound, depending upon the identities ofthe terminal groups. It is also believed that some of the existingterminal groups and/or some of the terminal groups created by chainscission of the silicone oil condense with other such terminal groups ofthe same or different silicone oil molecules to form siloxane bonds andevolve water, alcohol, or other compound. Ring structures, linearstructures (including those of increased molecular weight), and, if somesilicone oil having a functionality greater than two is also present,network structures may result. Irrespective of theory, a preferredembodiment is hydrophobic reinforced amorphous precipitated silica ofthe invention which comprises silicone oil or condensation residuethereof.

The reinforced amorphous precipitated silica of the present invention isgenerally such that when a uniform mixture of 100 parts by weightSilastic® Q4-2735 silicone gum (Dow Corning Corp. ), 40 parts by weightof the reinforced amorphous precipitated silica, 8 parts by weight ofSilastic® Q4-2737 Processing Aid (Dow Corning Corp.), and 0.5 part byweight of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane is sheeted out ata thickness of 2.03 millimeters, placed in a mold and cured under apressure of 10.3 megapascals and at a temperature of 170° C. for 10minutes, died out according to ASTM D 412-87 using Die C, post cured at250° C. for one hour, and tested for tensile strength according to ASTMD 412-87 using Method A, exhibits a tensile strength of at least 7.24megapascals. Specimens prepared for tensile strength testing in thismanner usually also exhibit a Durometer Shore A hardness according toASTM D 2240- 86 of at least 50. ASTM D 412-87 and ASTM D 2240-86 are, intheir entireties, incorporated herein by reference.

Another embodiment of the invention is an elastomeric compositioncomprising: (a) crosslinked poly(diorganosiloxane), and (b) reinforcedamorphous precipitated silica having, on a coating-free andimpregnant-free basis, a surface area of from about 220 to about 340square meters per gram, a pore diameter at the maximum of the volumepore size distribution function of from about 9 to about 20 nanometers,and a total intruded volume of from about 2.6 to about 4.4 cubiccentimeters per gram; the silica being distributed substantiallyuniformly throughout the crosslinked poly(diorganosiloxane).

The reinforced amorphous precipitated silica of the present invention isincorporated into a silicone elastomer gum in amounts sufficient toreinforce the silicone rubber when cured, i.e., reinforcing amounts.Usually the amount of precipitated silica used will range from about 10to about 100, in many cases from 20 to 70, preferably 30 to 60 parts oksilica per 100 parts of silicone elastomer, by weight.

As the silicone elastomer gum, there can be used any of the siliconegums known in the art which are cured, i.e., crosslinked, to a siliconerubber by means of a free radical generator. Examples of silicone gumsinclude the methyl, vinyl, phenyl, methyl vinyl, methyl phenyl andfluorinated silicone gums.

Free radical generators used to catalyze the curing (crosslinking) ofthe silicone gums are most frequently organic peroxides or gamma or highenergy electron radiation. Exemplary of the organic peroxides commonlyused are benzoyl peroxide, bis(2,4-dichlorobenzoyl) peroxide,2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, tert-butyl peroxybenzoate,and dicumyl peroxide. The peroxide is usually used in amounts of fromabout 0.2 to about 1 part per 100 parts of gum, by weight.

In addition to the precipitated silica and free radical generator, thesilicone gum can contain other additives such as processing aids (0 toabout 12 parts per 100 parts of gum, by weight), silane additive (0 toabout 1 part per 100 parts of gum, by weight), colorants, heatstabilizers, plasticizers, etc. The listing of optional ingredients isby no means exhaustive. These and other ingredients may be employed intheir customary amounts for their customary purposes so long as they donot seriously interfere with good polymer formulating practice. All theingredients except the peroxide are mixed in equipment such as a Banburyinternal mixer, a Sigma mixer, or a Baker-Perkins mixer until uniform.The mixture is then cooled, if necessary, and the peroxide initiator isadded. The mixture is then mixed to distribute the initiatorsubstantially uniformly throughout. The catalyzed mixture is then curedin a manner known to the art. Ordinarily, the mixture is cured at about170° C. for about 10 to about 15 minutes. Post cures of from about 1 toabout 4 hours at about 250° C. are often also used.

The invention is further described in conjunction with the followingexamples which are to be considered illustrative rather than limiting,and in which all parts are parts by weight and all percentages arepercentages by weight unless otherwise specified.

EXAMPLE I

An initial aqueous sodium silicate solution in the amount of 58.881liters was established in a reactor. The initial aqueous sodium silicatesolution contained about 2 weight percent SiO₂ and had an SiO₂ :Na₂ Omolar ratio of about 3.3. The initial aqueous sodium silicate solutionwas heated to 34° C. and over a period of 28 minutes and with agitation,26.708 liters of about 2 weight percent aqueous sulfuric acid was addedto the initial aqueous alkali metal silicate solution thereby toneutralize about 80 percent of the Na₂ O and to form a first reactionmixture. Over a period of 121 minutes, with agitation, and at atemperature of 80° C., a stream of 9.059 liters of additive aqueoussodium silicate solution containing about 13 weight percent SiO₂ andhaving an SiO₂ :Na₂ O molar ratio of about 3.3, and a stream of 15.321liters of about 4 weight percent aqueous sulfuric acid were addedsimultaneously to the first reaction mixture to form a second reactionmixture. The pH of the second reaction mixture was 9.1. A stream ofabout 8 liters of about 4 weight percent aqueous sulfuric acid was addedto the second reaction mixture with agitation at a temperature of 80° C.to form a third reaction mixture having a pH of 4.5. The third reactionmixture was aged with agitation at 80° C. for 30 minutes. The aged thirdreaction mixture was split into two approximately equal portions. Withagitation, 0.45 liter of additive aqueous sodium silicate solutioncontaining about 13 weight percent SiO₂ and having an SiO₂ :Na₂ O molarratio of about 3.3 was added to one portion of the aged third reactionmixture at 80° C. to form a fourth reaction mixture having a pH of 8.5.A fifth reaction mixture was formed by adding to the fourth reactionmixture with agitation and at a temperature of 80° C., 2.271 liters ofadditive aqueous sodium silicate solution containing about 13 weightpercent SiO₂ and having an SiO₂ :Na₂ O molar ratio of about 3.3 and byadding 5.5 liters of about 4 weight percent aqueous sulfuric acidsimultaneously to maintain the pH at about 8.5. The sequential additionsto form the fourth and fifth reaction mixtures were made over acollective time period of 43 minutes. The fifth reaction mixture wasaged with agitation at 80° C. for 45 minutes. With agitation, 1.5 litersof about 4 weight percent aqueous sulfuric acid was added to the agedfifth reaction mixture to form a sixth reaction mixture having a pH of4.5. The sixth reaction mixture was aged with agitation at 80° C. for 60minutes. The aged sixth reaction mixture was vacuum filtered using aseries of Buchner funnels. Just before air could be pulled through eachfilter cake, the addition of 16 liters of water to the funnel was begunfor the purpose of washing the filter cake. Air was briefly pulledthrough the washed filter cake. The wet filter cake contained 9.9percent solids by weight. After being removed from the funnels, the wetfilter cakes were stirred with a propeller type agitator to form a solidin liquid suspension. The suspension was dried in a Niro spray drier(inlet temperature about 360° C.; outlet temperature about 128° C.) toform a batch of dried reinforced precipitated silica. The product had asurface area of 333 square meters per gram, a pore diameter at themaximum of the volume pore size distribution function of 9 nanometers,and a total intruded volume of 3.21 cubic centimeters per gram. Theproduct was micronized in a fluid energy mill using compressed air asthe working fluid.

EXAMPLE II

An initial aqueous sodium silicate solution in the amount of 340.7liters was established in a reactor. The initial aqueous sodium silicatesolution contained about 2 weight percent SiO₂ and had an SiO₂ :Na₂ Omolar ratio of about 3.3. The initial aqueous sodium silicate solutionwas heated to 37° C. and over a period of 30 minutes and with agitation,2.449 liters of about 30 weight percent aqueous sulfuric acid and137.426 liters of water were added as separate streams to the initialaqueous alkali metal silicate solution to neutralize about 80 percent ofthe Na₂ O and to form a first reaction mixture. The first reactionmixture was heated with agitation to 95° C. During the heat-up 74.8liters of water was added. The diluted first reaction mixture was thenaged with agitation at 95° C. for 60 minutes. Over a period of 120minutes, with agitation, and at a temperature of 95° C., a stream of52.41 liters of additive aqueous sodium silicate solution containingabout 13 weight percent SiO₂ and having an SiO₂ :Na₂ O molar ratio ofabout 3.3 and a stream of 9.449 liters of about 30 weight percentaqueous sulfuric acid were added to the aged diluted first reactionmixture to form a second reaction mixture. The pH of the second reactionmixture was 9.1. A stream of about 8 liters of about 30 weight percentaqueous sulfuric acid was added to the second reaction mixture withagitation at a temperature of 95° C. to form a third reaction mixturehaving a pH of 4.5. The third reaction mixture was aged with agitationat 95° C. for 30 minutes. With agitation, 6.57 liters of additiveaqueous sodium silicate solution containing about 13 weight percent SiO₂and having an SiO₂ :Na₂ O molar ratio of about 3.3 was added to the agedthird reaction mixture at 95° C. to form a fourth reaction mixturehaving a pH of 8.7. A fifth reaction mixture was formed by adding to thefourth reaction mixture with agitation and at a temperature of 95° C.,30.18 liters of additive aqueous sodium silicate solution containingabout 13 weight percent SiO₂ and having an SiO₂ :Na₂ O molar ratio ofabout 3.3, and by adding liters of about 30 weight percent aqueoussulfuric acid as necessary to maintain the pH at about 8.7. Thesequential additions to form the fourth and fifth reaction mixtures weremade over a collective time period of 84 minutes. The fifth reactionmixture was aged with agitation at 95° C. for 45 minutes. Withagitation, 3.5 liters of about 30 weight percent aqueous sulfuric acidwas added to the aged fifth reaction mixture to form a sixth reactionmixture having a pH of 4.5. The sixth reaction mixture was aged withagitation for 60 minutes maintaining 95° C. and thereafter for about 900minutes without temperature maintenance. The temperature at theconclusion of the 900 minute period was 66° C. The aged sixth reactionmixture was filtered in a filter press. The filter cake was washed withwater until the conductivity of the filtrate had dropped to 90micromhos/cm. The wet filter cake and added water were mixed with aCowles blade to form a solid in liquid suspension containing 9.7 percentsolids by weight. The suspension was dried in a Niro spray drier (inlettemperature about 360° C.; outlet temperature about 128° C.) to form thereinforced precipitated silica product. The product had a surface areaof 232 square meters per gram, a pore diameter at the maximum of thevolume pore size distribution function of 14 nanometers, and a totalintruded volume of 3.09 cubic centimeters per gram. The product wasmicronized in a fluid energy mill using compressed air as the workingfluid.

EXAMPLE III

An initial aqueous sodium silicate solution in the amount of 41314liters was established in a reactor. The initial aqueous sodium silicatesolution contained about 2 weight percent SiO₂ and had an SiO₂ :Na₂ Omolar ratio of about 3.2. The initial aqueous sodium silicate solutionwas heated to 34° C. and over a period of 33 minutes and with agitation,1086 liters of about 30 weight percent aqueous sulfuric acid and 11356liters of water were added to the initial aqueous alkali metal silicatesolution to neutralize about 80 percent of the Na₂ O and to form a firstreaction mixture. The first reaction mixture was heated with agitationto 95° C. over a period of about 2 hours. The first reaction mixture wasthen aged with agitation at 95° C. for 65 minutes. A total of 2557liters of water were added during the heating and aging periods. Over aperiod of 119 minutes, with agitation, and at a temperature of 95° C., astream of 6314 liters of additive aqueous sodium silicate solutioncontaining about 12.6 weight percent SiO₂ and having an SiO₂ :Na₂ Omolar ratio of about 3.2, a stream of 1124 liters of about 30 weightpercent aqueous sulfuric acid, and a stream of 549 liters of water wereadded simultaneously to the first reaction mixture to form a secondreaction mixture. The pH of the second reaction mixture was 9.6. Astream of about 777 liters of about 30 weight percent aqueous sulfuricacid and a stream of 117 liters of water were added to the secondreaction mixture with agitation at a temperature of 95° C. to form athird reaction mixture having a pH of 4.5. The third reaction mixturewas aged with agitation at 95° C. for 30 minutes during which period 46liters of water was added. With agitation, water and 890 liters ofadditive aqueous sodium silicate solution containing about 12.6 weightpercent SiO₂ and having an SiO₂ :Na₂ O molar ratio of about 3.2 wasadded to the aged third reaction mixture at 95° C. to form a fourthreaction mixture having a pH of 8.5. A fifth reaction mixture was formedby adding to the fourth reaction mixture with agitation and at atemperature of 95° C., water and 3528 liters of additive aqueous sodiumsilicate solution containing about 12.6 weight percent SiO₂ and havingan SiO₂ :Na₂ O molar ratio of about 3.2 and by adding 846 liters ofabout 30 weight percent aqueous sulfuric acid as necessary to maintainthe pH at about 8.5. The sequential additions to form the fourth andfifth reaction mixtures were made over a collective time period of 80minutes. The fifth reaction mixture was aged with agitation at 95° C.for 45 minutes. With agitation, water and 259 liters of about 30 weightpercent aqueous sulfuric acid were added to the aged fifth reactionmixture to form a sixth reaction mixture having a pH of 4.5. A total of568 liters of water was added during formation of the fourth through thesixth reaction mixtures. The sixth reaction mixture was aged withagitation and without temperature maintenance for 653 minutes. The finaltemperature was 82° C. The aged sixth reaction mixture was divided intotwo batches of about 40504 liters and 39747 liters, respectively. Eachbatch was filtered in a filter press. The filter cakes were washed withwater until the conductivity of the filtrate had dropped to about 5micromohs/cm. A portion of the washed filter cakes from the first filterpress batch was removed and set aside. The remainder of the washedfilter cakes and added water were mixed with a Cowles blade to form asolid in liquid suspension containing 12 percent solids by weight. Thesuspension was dried in a Bowen spray drier (inlet temperature about620° C.; outlet temperature about 130° C.) to form the reinforcedprecipitated silica product. The product had a surface area of 236square meters per gram, a pore diameter at the maximum of the volumepore size distribution function of 15 nanometers, and a total intrudedvolume of 3.2 cubic centimeters per gram.

EXAMPLE IV

Filter cakes set aside from the first filter press batch in Example IIIwere added to a Cowles liquifier along with water to produce 1181 litersof a 10 weight percent solids suspension; this is equivalent to 124.4kilograms of dry weight silica. While agitating, 19.3 kilograms ofPetrarch CPS340 silanol terminated polydimethylsiloxane (averagemolecular weight 400-700; kinematic viscosity of 15-35 centistokes;functionality is 4-6 weight percent) (Petrarch Systems) was added toform a first batch of feedstock. This procedure was repeated to form asecond batch of feedstock. Both batches were combined. The combinedbatches were spray dried as described in Example III.

A portion of the spray dried powder was micronized in a fluid energymill using compressed air as the working fluid. The micronized powderwas put into trays and heat-treated in a circulating air oven at 270° C.for 16 hours to produce a first hydrophobized reinforced amorphousprecipitated silica (HRAPS 1).

The remainder of the spray dried powder was micronized in a Jet-O-MizerModel 0405-C fluid energy mill (Fluid Energy Processing & Equipment Co.)using steam as the working fluid to produce a second hydrophobizedreinforced amorphous precipitated silica (HRAPS 2). The temperature ofthe injected steam was about 275° C. and the outlet temperature wasabout 130° C.

After cooling to about room temperature, HRAPS 1 and HRAPS 2 were eachseparately compounded with a commercial silicone elastomeric gum and acommercial processing aid. Blending was done in a Baker-Perkins mixer.After massing, each mixture was heat treated at about 150° C. for aboutone hour under mild vacuum. The mixtures were then aged at ambientconditions at least 24 hours. Each mixture was then put on a two-rollrubber mill and initiator and thermal stabilizer were added. Eachmixture was then sheeted out and pressed into a test slab about 2.03millimeters thick and cured for 10 minutes at 170° C. under a pressureof 10.3 megapascals. Dumbbell specimens were died out according to ASTMD412-87, Die C, and hung in a circulating air oven at 250° C. for onehour. The resulting post-cured specimens were tested according to ASTMD2240-86 for durometer Shore A hardness and according to ASTM D412-87,Method A, for tensile strength and percent elongation at break.Comparative samples were prepared in a similar manner using fumed silicainstead of the hydrophobized reinforced precipitated silicas describedabove. The components and their amounts are shown in Table 1 and thetest results are shown in Table 2.

                  TABLE 1                                                         ______________________________________                                        Sample No.        1         2      3                                          ______________________________________                                        Components, parts by weight                                                   Silicone Gum (1)  100       100    100                                        Processing Aid (2)                                                                              4         4      8                                          HRAPS 1 (3)       46        0      0                                          HRAPS 2 (3)       0         46     0                                          Fumed Silica (4)  0         0      40                                         Initiator (5)     0.5       0.5    0.5                                        Thermal Stabilizer (6)                                                                          1         1      1                                          ______________________________________                                         (1) Silastic ® Q42735 poly(dimethylsiloxane) (Dow Corning Corp.) with     vinyl groups attached.                                                        (2) Silastic ® Q42737 Processing Aid (Dow Corning Corp.)                  (3) This is equivalent to about 40 parts of silica on a dry weight            basis.(4)                                                                     (4) CabO-Sil ® MS75 fumed silica (Cabot Corp.)                            (5) 2,5dimethyl-2,5-bis(tert-butylperoxy)hexane                               (6) Silastic ® HT1 Heat Stabilizer (Dow Corning Corp.)               

                  TABLE 2                                                         ______________________________________                                        Sample No.     1          2       3                                           ______________________________________                                        Tensile Strength, MPa                                                                        9.05       9.07    9.26                                        Elongation, percent                                                                          386        387     393                                         Durometer, Shore A                                                                           60         66      57                                          ______________________________________                                    

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except insofar as they are included in the accompanyingclaims.

We claim:
 1. An elastomeric composition comprising:(a) crosslinkedpoly(diorganosiloxane), and (b) reinforced amorphous precipitated silicahaving, on a coating-free and impregnant-free basis, a surface area offrom about 220 to about 340 square meters per gram, a pore diameter atthe maximum of the volume pore size distribution function of from about9 to about 20 nanometers, and a total intruded volume of from about 2.6to about 4.4 cubic centimeters per gram;said silica being distributedsubstantially uniformly throughout said crosslinkedpoly(diorganosiloxane).
 2. The elastomeric composition of claim 1wherein said surface area is from about 220 to about 300 square metersper gram.
 3. The elastomeric composition of claim 1 wherein said surfacearea is from about 220 to about 270 square meters per gram.
 4. Theelastomeric composition of claim 1 wherein said pore diameter at themaximum of the volume pore size distribution function is from about 13to about 18 nanometers.
 5. The elastomeric composition of claim 1wherein said total intruded volume is from about 3 to about 4.4 cubiccentimeters per gram.
 6. The elastomeric composition of claim 1 whereinsaid silica contains less than about 0.5 percent by weight alkali metalsalt.
 7. The elastomeric composition of claim 1 wherein said silicacontains less than about 0.2 percent by weight alkali metal salt.
 8. Theelastomeric composition of claim 1 wherein:(a) said pore diameter at themaximum of the volume pore size distribution function is from about 13to about 18 nanometers; and (b) said total intruded volume is from about3 to about 4.4 cubic centimeters per gram.
 9. The elastomericcomposition of claim 8 wherein said surface area is from about 220 toabout 300 square meters per gram.
 10. The elastomeric composition ofclaim 8 wherein said surface area is from about 220 to about 270 squaremeters per gram.